In many industrial processes materials in different states flow or are stored in containers, pipeworks or similar, in which case it is necessary in the process to know for example amounts, flow rates, mixture composition and similar information of different materials. Such processes typically develop boundaries between different materials, wherein the boundaries can be defined such that the densities of the materials on different sides of the boundary are different. In practice, a boundary has a specific transition area where the physical properties of the material are altered. One example of boundaries is a mixture of oil and water, where oil, being a lighter material, forms a layer on the surface of water, and a distinct boundary is found between these materials. Another example can be to examine different soil layers and sedimentation of materials in the earth, where interesting boundaries in the breaking of ore include for example boundaries between the rock material including precious metals and other rock material.
In many cases it is necessary to know the amount of material in a container or area to be examined. These situations occur in particular in the ore preparation process and sewage disposal process. A particular application in the preparation of ore is the thickening machine of the process. When the materials are distinctly separated and there is a distinct boundary between them the separation or thickening of the materials can take place.
The location of a boundary examined for example in the direction of height of the container to be examined is particularly interesting information for a number of processes. In general, these boundaries located for example in a container or in the earth are practically impossible to observe by visual inspection. On this account, more developed methods to detect boundaries are needed. There may be many different boundaries which may exist between different states of materials; however, the boundaries between two liquids or a liquid and a solid are particularly discussed below.
Boundaries between different materials precipitated by layers or the height of a fluid level have been measured in the prior art for example by acoustic and optical methods and methods based on gravity (pressure measurements) and electrical measurements. These methods have been described in [1] J. Vergouw, C. O. Gomez, J. A. Finch: “Estimating true level in a thickener using a conductivity probe”, Minerals Engineering, 17:87-88, 2004; [2] O-P Tossavainen, M. Vauhkonen, V. Kolehmainen: “A three-dimensional shape estimation approach for tracking of phase interfaces in sedimentation processes using electrical impedance tomography”, Measurement Science and Technology, 18:1413-1424, 2007 and [3] M. Maldonado, A. Desbiens, R. del Villar: “An update on the estimation of the froth depth using conductivity measurements”, 2008. One known method of measuring boundaries by acoustic waves in based on reflection from a discontinuity point. A sound wave transmitted to a material under examination is reflected from the boundary as an electromagnetic wave would reflect from the boundary of an impedance variation. By calculating the propagation times of the reflected wave it is possible to calculate the distance of the boundary from the transmitter and further the desired height of the boundary in the y-dimension.
EIT (Electrical impedance tomography), in turn, is a method in which electrodes can be mounted on the surface of an object to be examined. The basic principle of the method is that a set of electrodes is mounted on the surface of the study object and fed with minute alternating current, whereafter the potential differences, i.e. voltages, between the electrodes are measured. Typically, the voltage measurement is made from the same electrodes as the current feed. EIST (Electrical impedance spectroscopy tomography), in turn, means that a number of different frequencies are used in the measurement, i.e. the measurements are typically made over a desired continuous frequency band. From the measured potential differences with a number of different electrode intervals it can be concluded that the electrical conductivity or permittivity of the object to be measured varies as a function of location, provided that the object in question is not completely homogenous. In practice, the conductivities are calculated by various mathematical methods in which suitable calculating models can be utilized. Such a calculation relates to the field of inversion calculation. Finally, for example a sectional view of the level of the measured object on which the electrodes have been disposed is obtained from the electrical conductivities as a function of location.
In the prior art, the height of the boundary between a solid material precipitated on the bottom of a container under examination and a liquid on top of it has been measured by introducing a measurement sensor disposed at one end of an arm directly in the precipitate. This in conjoined with fouling of the sensor, which considerably affects the measuring accuracy and performance of the sensor. In addition, the lifetime of the sensor becomes in this case, at the worst, very short.
In the prior art, sensors have been used with the electrodes disposed in a single dimension, i.e. using a straight arm with electrodes at one or both ends. For example minute electric current has been fed via such a pair of electrodes, and the potential difference, i.e. the voltage, has been measured between these electrodes. By placing the sensor arm in different locations for example in a fluid container and examining the voltage variation between the different locations it has been possible to obtain information of possible boundaries between two different materials. However, this has required numerous reproducible measurements, and fouling or even breakdown of the sensors is an essential problem.
More specifically, the boundary has been measured in the prior art as follows. Consider a container with solid precipitated material on the bottom in a layer of a specific height, and for example water on top of the precipitate. The boundary between the precipitate and water is assumed to be distinctly defined, i.e. it can be assumed that under examination in the height direction a leap in the properties of the material occurs at one point (a single coordinate value in the y-dimension). Consider an arrangement where a pair of electrodes is disposed at each end of a straight pipe. The pipe is placed vertically in the container such that the lower pair of electrodes is entirely within the precipitate and the upper one is entirely within the water. It is further assumed herein that the situation is not dynamic, i.e. the boundary between the liquid and the precipitate remains stably immovable.
According to the principle of impedance tomography, minute electric current is fed to the electrodes, and the voltage is measured on these electrodes. In other words, the voltage is measured between two adjacent electrodes. From the measurement results it is easy to calculate the characteristic resistivity of the material surrounding each pair of electrodes by formulae:
                              R          1                =                                                            U                1                                            I                1                                      ⁢                                                  ⁢            and            ⁢                                                  ⁢                          R              2                                =                                    U              2                                      I              2                                                          (        1        )            
From these values the electrical conductivities σ1 and σ2 of each material can be calculated. If the electrical conductivities σ1 and σ2 differ from each other, it can be concluded that the boundary of the materials is within the region between the two pairs of electrodes. By performing a new measurement by raising or lowering the pipe in the vertical direction a new estimate for the height is obtained after the above-mentioned calculations.
The main problem of the prior art has been fouling of the sensors and the resulting loss of operational reliability, as well as slowness in determining the boundary as a result of reproduction of the measurements. Furthermore, the sensors can typically be used in the working order for a moment, but their lifetime is not long. From the fouling it follows that the sensors or measurement electrodes must be cleaned relatively often, which means from the process point of view implementation of an automatic cleaning system for the system or, alternatively, maintenance personnel must manually clean or service the apparatus on a regular basis, which further results in pauses in the use of the apparatus.